1. Field of the Invention
The present invention relates to a laser shock hardening method and apparatus for irradiating the surface of a solid material, such as a metal or a ceramic, with a pulsed laser beam through a liquid to adjust surface or internal characteristics of the material, such as structure, hardness and residual stress.
2. Background Art
Defects in a structure, such as corrosion and cracks, in most cases originate from the surface, and the life of the structure depends on its surface characteristics. Attempts have therefore been made to improve the mechanical or chemical properties of the surface of a material, such as fatigue strength, corrosion resistance and wear resistance, thereby prolonging the life of the structure.
Shot peening is a typical surface processing technique. This technique enables a rise in the hardness of a surface layer of a workpiece and introduction of compressive residual stress into the surface layer, and therefore is widely used in the industrial fields of automobiles, aircrafts, etc. (see, for example, “Metal Fatigue and Shot Peening”, edited by Society of Shot Peening Technology of Japan).
Laser irradiation, on the other hand, enables precise and high-speed control of energy density and irradiation point and can carry out high-speed processing, rapid heating/quenching processing, etc. which are difficult with other methods. Accordingly, various laser irradiation techniques have been developed which find wider application to processing of materials.
One such technique is laser shock hardening which involves irradiation of the surface of a material with a pulsed laser beam through a liquid. As with shot peening, this technique enables a rise in the hardness of a surface layer of a workpiece and introduction of compressive residual stress into the surface layer.
Laser shock hardening has a higher effect than shot peening and, in addition, has various excellent advantages that shot peening does not have, such as capability of contactless operation, no involvement of reaction force and capability of precise control of laser irradiation conditions and laser irradiation sites. Development and practical application of this processing method are now under way (Japanese Patent Laid-Open Publications Nos. 7-246483, 8-112681, 8-326502, and 2003-504212).
Laser shock hardening, which involves irradiating the surface of a solid material, such as a metal or a ceramic, with a pulsed laser beam through a liquid to adjust surface of internal characteristics of the material, such as structure, hardness and residual stress, will now be described with reference to FIGS. 1 through 3.
FIG. 1 illustrates a method in which a workpiece 41 disposed in a liquid 22 is irradiated with a pulsed laser beam 51 to adjust the material characteristics, such as structure, hardness and residual stress, of the workpiece 41.
When the peak power density of the laser beam 51 exceeds the plasma generation threshold of the workpiece 41 (approximately 0.1 to 10 TW/m2 in the case of a metal), the topmost surface layer (1 μm or lower) of the workpiece 41 evaporates instantly to generate a plasma 52. Because of inertia strongly acting instantaneously in the liquid 22, the plasma 52 can little expand and the energy of laser beam 51 concentrates in a narrow area. Accordingly, the pressure of the plasma can even reach 10-100 times the pressure in the air or in vacuum.
When water is used as the liquid 22, the pressure P (GPa) of the plasma generated is approximately equal to (0.2×I)0.5, wherein I (TW/m2) represents the peak power density of the laser beam 51 applied to the workpiece 41. In case the liquid 22 is a liquid other than water, such as an alcohol, ammonia water or a boric acid solution, the plasma pressure can be determined by the equation: P=(0.2×I×k)0.5, k=(acoustic impedance of liquid)/(acoustic impedance of water)
The “acoustic impedance of liquid” is equal to (density of liquid)×(sonic velocity in liquid). With the above-described liquid other than water, therefore, the plasma pressure in the liquid does not differ significantly from that in water. Thus, in either case, when the size and the pulse energy of the laser beam 51 are so controlled as to make the peak power density of the laser beam 51 1-100 TW/m2 at the surface of the workpiece 41, the pressure of the plasma 52 will be approximately 450 MPa-4.5 GPa.
The high-pressure plasma 52 thus generated instantaneously compresses the surface of the workpiece 41 and the surface displacement caused by the compression generates a shock wave 53 that propagates in the depth direction of the workpiece. The shock wave 53, when its pressure exceeds the yield stress of the workpiece, will cause a local plastic deformation. This makes it possible to adjust the material characteristics, such as structure, hardness and residual yield.
FIGS. 2 and 3 illustrate an example of adjustment of material characteristics by laser shock hardening, FIG. 2 showing a change in the hardness of a stainless steel (SUS 304) and FIG. 3 showing a change in the residual stress of the stainless steel. A laser beam of a pulse energy of 200 mJ and a pulse width of 8 ns was collected such that the irradiation spot takes the shape of a circle having a diameter of 0.8 mm and was applied at 36 pulses per 1 mm2, so that the peak power density became 50 TW/m2. Reference numerals 71, 72 denote the hardness values before and after processing. The comparative data shows a rise in the hardness in the region nearly to the depth of 1 mm by the laser shock hardening processing. Reference numerals 73, 74 denote the residual stress values before and after processing. The comparative data shows an improvement from tensile to compressive in the residual stress in the region nearly to the depth of 1 mm by the laser shock hardening processing.
Such a rise in the hardness of the surface of a material and the formation of a compressive residual stress are effective in enhancing fatigue strength and preventing stress corrosion cracking. Therefore, laser shock hardening has been progressively employed in the aircraft industry, the automobile industry, the atomic industry, etc.
Since laser shock hardening involves direct irradiation of the surface of the workpiece 41 with the pulsed laser beam 51, there is a case where an element, constituting the liquid 22 decomposed by the plasma 52, reacts with the surface of the workpiece 41.
For example, in the case of laser shock-hardening a stainless steel in a water atmosphere, hydrogen and oxygen are generated by the decomposition of water, and the oxygen reacts with the surface of the stainless steel, whereby a strong black oxide film having a thickness of about 1 μm, composed mainly of Fe3O4, is formed on the surface after the processing.
In case such a black film is undesirable for its appearance, a coating film having a thickness of the order of several tens of μm may be formed on the surface of the workpiece 41, for example with a paint or a metal tape, prior to laser shock hardening. After removing the coating film, the surface state of the workpiece 41 will be almost the same as that before the processing.
In laser shock hardening, the surface of a material is irradiated with a pulsed laser beam through a liquid, such as water. Upon irradiation with a laser beam, there occurs the phenomenon that a high-pressure plasma, generated on the surface of the material, spatters the liquid and disturbs the liquid surface. When irradiation with the next laser beam is carried out shortly thereafter, the position or the shape of the irradiation spot can change due to refraction. The next laser beam irradiation should therefore be awaited until the disturbance of the liquid surface settles down, which precludes speeding up of the processing.
Further, laser shock hardening is generally carried out by applying a pulsed laser beam, shaped into a circular or square shape of a size of about 1 to several mm, to the surface of a material. Laser shock hardening thus has the drawback that only a small area can be processed with one pulse, that is, the processing speed is low. Studies have therefore been made on methods for speeding up of processing, for example, the use of a laser oscillator with a high repetition or the use of a laser oscillator with a large pulse energy. Such speeding-up methods, however, entail such problems as the necessity of using a larger-sized laser oscillator or a larger-sized driving device for moving a workpiece or an irradiation head. Speeding up of laser shock hardening processing has thus been difficult.